Amino acid transport across blood-aqueous barrier of mammalian species

Abstract

The concentration of free amino acids in the plasma, aqueous and vitreous humors of sheep was determined by ion exchange chromatography. With the possible exception of methionine and glycine, the concentration of every amino acid in the aqueous humors of posterior and anterior chamber was found to be significantly higher than in the corresponding plasma while the vitreous humor was deficient in all amino acids with respect to these fluids. The most pronounced concentration gradients between aqueous humors and plasma were noted for aspartic acid and glutamic acid, which were found to be 9–13 times higher in the aqueous humors. Cystine and lysine, which were least concentrated in the aqueous humors, were approximately 1·7 times that in the corresponding plasma; for all other compounds the aqueous/plasma ratios were greater than 2. These observations lead to the conclusion that most of the amino acids are transported across the ciliary epithelium by an active process. While the concentration gradients between the anterior aqueous and plasma were similar to those existing between the aqueous of posterior chamber and plasma for all amino acids, the relative concentration of arginine, aspartic acid, glutamic acid, histidine, serine and threonine in the anterior chamber with respect to plasma appeared to be higher than in the posterior chamber, suggesting the possibility that there may also be active transport across the iris. The role of vitreous humor in regulating the amino acid levels in the aqueous humors was investigated in rats by abolishing the vitreous/plasma gradient. Following a single intravenous injection of sodium iodate the steady state concentration of [ 14C]cycloleucine in the vitreous increased from 34% of plasma to 90% of plasma value while that in the anterior aqueous increased from 80 to 120% of plasma level. The results suggest that cycloleucine and possibly other amino acids are actively transported across the blood-aqueous barrier of these animals. It is concluded that the deficiency of amino acids observed in the aqueous humors of certain species may be due to the “sink effect” of the vitreous humor and that active transport of amino acids across the blood-queous barrier may be involved in all mammalian species.